Models used to estimate past ocean temperatures might be based on a flawed assumption, according to new research.

If true, it would mean our ancient seas were far cooler than previously calculated, and our planet’s current warming trend is even more extraordinary than we thought.

A team of scientists from some of Europe’s leading research institutes has taken a critical look at a chemical process that has served as a proxy for determining the temperatures of oceans millions of years in the past.

Even the most solid of scientific models rests on fairly well-reasoned assumptions.

In this case, the method for calculating temperature was based on the thought that temperatures were preserved perfectly inside tiny marine organisms called foraminifera.

Specifically, the exact ratio of oxygen-18 to oxygen-16 in the calcite of the organisms’ exoskeletons varies with the isotope concentrations in the environment – a factor that was determined by things like acidity and salinity – and the water’s temperature.

So if we determine the differences in the oxygen isotopes in fossils, we have a record of the temperatures as they were when they lived a little over 100 million years ago.

This tells us the temperature of the deeper parts of the ocean at the tropics were about 15 degrees Celsius warmer than today.

Yet it turns out things might not be quite so straightforward.

“What appeared to be perfectly preserved fossils are in fact not,” says Sylvain Bernard, a mineralogist from the French National Center for Scientific Research.

Evidence now suggests the ratio of oxygen-18 to oxygen-16 in the buried marine life might not be quite as stable as thought.

To test how the chemistry of the calcite in the foraminifera’s shells might continue to change over time, the researchers placed a sample of the organisms in artificial sea water that contained just isotopes of oxygen-18.

They then cranked the temperature to simulate the heat generated by being buried beneath a pile of sediment and used a device called a nanoscale secondary ion mass spectrometer (or NanoSIMS) to analyse changes in the calcite’s oxygen ratios.

Sure enough, the equilibrium shifted, changing the ratios.

“This means that the paleotemperature estimates made up to now are incorrect,” says Bernard.

Taken at face value, it implies the waters probably weren’t all that much warmer than today. The discovery also helps resolve a paradox that has hinted at an inconsistency in the most favoured models.

Using the oxygen isotope method, ocean temperatures in the tropics during the warm Cretaceous period weren’t all that different to the surface temperatures at the poles.

Unfortunately other models on climate and ocean currents don’t gel with this shallow gradient, hinting at a problem.

In addition, analysing magnesium isotopes in the foraminifera fossils instead of oxygen suggests the sea surface temperatures at higher latitudes were also colder than estimated.

For all of the changes our planet has experienced over the past 100 million years, our oceans have remained pretty stable as far as temperature goes. Cast in that light, today’s rapid global warming trend is even more dramatic than we’d thought.

The next step for researchers is to go back to the drawing board on existing data and see exactly what difference the changes make to historical records.

“To revisit the ocean’s paleotemperatures now, we need to carefully quantify this re-equilibration, which has been overlooked for too long,” says geochemist Anders Meibom from the Swiss Federal Institute of Technology in Lausanne.

“For that, we have to work on other types of marine organisms so that we clearly understand what took place in the sediment over geological time.”

We truly are living in an extraordinary period of our planet’s history.